Thrombin is an important serine protease in hemostasis and thrombosis. One of the key actions of thrombin is receptor activation. A functional human thrombin receptor, cloned by Coughlin in 1991 (T.-K. Vu, Cell 1991, 64, 1057), was found to be a member of the G-protein coupled receptor (GPCR) superfamily. The receptor activation putatively occurs by N-terminal recognition and proteolytic cleavage at the Arg-41/Ser-42 peptide bond to reveal a truncated N-terminus. This new receptor sequence, which has an SFLLRN [SEQ. ID. NO:1] (Ser-Phe-Leu-Leu-Arg-Asn) N-terminus acting as a tethered ligand to recognize a site on the receptor, can trigger activation and signal transduction leading to platelet aggregation. Since 1991, two other protease-activated receptors with extensive homology to the thrombin receptor, "PAR-2" (S. Nystedt, Proc. Natl. Acac. Sci USA 1994, 91, 9208) and "PAR-3" (H. Ishihara, Nature 1997, 386, 502), were cloned, and found to be activated by similar N-terminal hexapeptide sequences. Thrombin receptor (PAR-1) specific antibody-induced blockade of the platelet thrombin receptor has shown efficacy against arterial thrombosis in vivo (J. J. Cook Circulation 1995, 91, 2961). Hence, antagonists of the thrombin receptor based on SFLLRN [SEQ. ID. NO:1] are useful in antagonizing these protease-activated receptors and as such may be used to treat platelet mediated thrombotic disorders such as myocardial infarction, stroke, restenosis, angina, atherosclerosis, and ischemic attacks by virtue of their ability to prevent platelet aggregation.
The thrombin receptor has also been identified on other cell types: endothelial, fibroblast, osteosarcoma, smooth muscle, and neuronal/glia. Thrombin activation of endothelial cells upregulates P-selectin to induce polymorphonuclear leukocyte adhesion--an inflammatory response of the vessel wall (Y. Sugama, J. Cell Biol. 1992, 119, 935). In fibroblasts, thrombin receptor activation induces proliferation and transmission of mitogenic signals (D. T. Hung, J. Cell Biol. 1992, 116, 827). Thrombin has been implicated in osteoblast proliferation through its activation of osteoblast cells (D. N. Tatakis, Biochem. Biophys. Res. Commun. 1991, 174, 181). Thrombin has been implicated in the regulation and retraction of neurons (K. Jalink, J. Cell. Biol. 1992, 118, 411). Therefore, in this context, the antagonist compounds of this invention may also be useful against inflammation, restenosis, osteoporosis, and neurodegenerative disorders.
The compounds of the present invention are azole peptidomimetics represented by the general formula (I) below. Azole-containing cyclic peptides have been synthesized to be employed as cytotoxic agents (C. Boden, Tetrahedron Lett. 1994, 35, 8271). By contrast, the azole peptidomimetics of the present invention are strictly acyclic with activity against the thrombin receptor. Azole-based dolastatin analogues have been prepared as antitumor agents (K. Sakakibara, PCT Int. Appl., 31 pp., WO9633212). These compounds contain a 4-thiazole-alkylamide C-terminus, whereas the compounds of the present invention require at least two amino acid residues C-terminal to the 4-thiazole carboxamide for activity against the thrombin receptor. Similarly, azole endothelin antagonists have been prepared which contain a 4-thiazole-carboxylic acid C-terminus (T. von Geldern, J. Med. Chem. 1996, 39, 957).